Depends on the way they will be played back - headphone mixes might not sound good played from loudspeakers, and vice versa.
Headphones tend to be brighter, and they direct the sound straight to the eardrums.
It's good to, when mixing, at least check your mix with monitors a couple of times to make sure it sounds good there too - especially to check bass.
It's also important to know the tendencies of your headphones - if they tend to be bass-heavy, for example, you need to not let yourself get tricked into killing the bass, which is another reason that checking with monitors is good.

(http://www.bedroom-recording.com/headphones.html)

Headphones (or cans, as they're sometimes called) have totally different sound than a set studio monitors. The whole audio spectrum is different, largely because of the distance from the speaker to your ear. Even near field monitors are 2-4 feet away, while heaphone speakers are less than 2 inches.

Because of this the tonal qualities will be different. The music will sound "up there", or right at you. Because the speakers are right beside your ear, a track panned to center will seem to come from inside your head, not in front of you.

By putting the sound so close to your ear, headphones will let you listen more critically to the mix. Some eliminate external sound, and they all let you listen to the more subtle dynamics and other possible background noises that might've been introduced during recording.

Since the speakers are so close to your ears, the sound doesn't need to be nearly as loud as near field monitors. The cones are much smaller, like 1 or 2 inches instead of 5 to 8 inches. (Just imagine an 8 inch speaker mounted in a set of phones!)

But by being smaller, the bass response is limited. They do well at covering it up and sounding good at the lower frequencies, but you will never get the bass you can "feel". I make up for that by leaving my monitors on so it fills out the low end, but doing the critical listening with the cans.